The Zika virus is transmitted by Aedes species mosquitoes, which also spread dengue and chikungunya viruses. Zika causes a mild illness with fever, rash, joint pain and conjunctivitis, with symptoms usually lasting under a week.

But in pregnant women, the virus can spread to the foetus and cause microcephaly - a condition involving brain shrinkage that severely limits a child's intellectual and physical development - or death.

Brazil has been the hardest hit, documenting more than 3,500 cases of microcephaly between October 2015 and January 2016

Health experts are unsure why the virus, which was first detected in Africa in 1947 but unknown in the Americas until last year, is spreading so rapidly in Brazil and neighbouring countries.

Last week, the United States warned pregnant women to avoid travelling to 14 countries and territories in the Caribbean and Latin America owing to the virus.